Footwear, in particular athletic footwear, are expected to provide proper shock absorption and stability thereby preventing potential harmful effects of vigorous movements such as running and jumping on the wearer's feet. The footwear industry has been developing athletic shoes in an effort to maximize shock absorption and stability while also maximizing comfort and durability. Unfortunately, these goals are potentially in conflict with each other. For example, a shoe that provides adequate shock absorption and comfort may not provide sufficient stability. To further advance the development of athletic shoes, a basic understanding of the dynamics of running and the mechanisms of running injuries is important.
A typical walking or running gait cycle involves two phases: (1) a stance phase, and (2) a swing phase. One foot contacts the support surface such as the ground and bears weight in the stance phase while the other foot is moving through the air and advances in the swing phase. The two phases are repetitive. The difference between the running and walking gait cycles is that at one point during the running cycle the person is airborne without bearing any weight, whereas the walking cycle does not include such an airborne moment.
The stance phase of a running gait cycle may be further divided into three periods: (1) the loading period, also called “the impact and support period” or “the heel strike period,” (2) the mid-stance period, also called “the mid-stance and propulsion period,” and (3) the toe-off period, also called “the recovery period.” For a typical runner of a heel-to-toe running style, the loading period begins with a first contact of the heel with the running surface, followed by a controlled lowering of the forefoot to the running surface. The first contact of the heel typically occurs at the rear, outer part of the heel. The mid-stance period begins once the forefoot is in contact with the running surface. During the mid-stance period, the contraction of the musculature of the leg generates power to propel the body forward. The heel progressively lifts and the forefoot flexes at the metatarsophalangeal joint. Then in the toe-off period, the foot disengages contact with the running surface and the foot becomes airborne.
Pronation is a normal movement of the foot that occurs during the loading and mid-stance periods of the stance phase of the gait cycle. At heel strike during the loading period, the heel of the foot is supinated and makes initial contact with the running surface as described earlier. Instantaneously, the joint between the foot bones called the subtalar joint is unlocked, allowing pronation, a coordinated triplane motion of the foot, to occur during the forefoot lowering events of the loading period of the stance phase. The coordinated triplane motion of the foot involves three planes of motion: (1) abduction, in which the front of the foot is turned outwards and away from the line of progression of the runner; (2) dorsiflexion, in which the front of the foot is angled upwards relative to the heel of the foot; and (3) eversion, in which the sole of the foot is turned outward relative to the heel of the foot. With the combination of these three motions, the foot rolls from the outside or lateral side to the inside or medial side of the foot resulting in the medial aspect (the arch area) of the foot coming into contact with the running surface, thus allowing the foot to adapt to the running surface and to transfer some of the loading force to the running surface, thereby reducing the risk of injury during the stance phase of running. The pronated position of the foot is maintained throughout the mid-stance period.
Supination typically follows pronation. As the body moves forward over the foot, the subtalar joint locks. This allows a reversal of the events that have occurred during the loading period to occur during the mid-stance period. Supination is a coordinated triplane motion of the foot, which involves three planes of motion: (1) adduction, in which the locking of the subtalar joint allows the foot to turn inward toward the line of progression; (2) plantarflex, in which the forefoot is flexed downward relative to the heel; and
(3) inversion, in which the sole of the foot is turned inward relative to the heel. With the combination of these three motions, the foot continues rolling forward onto the toes. During motion through ball and toe contact, the foot rolls outward just before the toes starts to leave the ground. The combination of these motions allows the foot to be converted from a mobile adaptor to a rigid lever, which is essential for the forward propulsion of the body. The foot remains supinated while it is off the ground between steps.
Although pronation is a natural action and is considered an important and healthy response to the intense amount of shock imposed upon the foot, excessive pronation and high pronation velocity have been suggested by biomechanists to cause a variety of injuries at the ankle, knee and hip among runners and other athletes. Many prior art soles have been designed to control pronation and supination. However, as the stability of the sole increases to control the amount of lateral motion of a foot in order to prevent excessive pronation, the shock absorption properties for reducing the impact of strike forces on the foot usually decrease. Thus, the footwear industry continues to seek a proper balance between the stability and shock absorption properties in designing shoe soles.
For Example, U.S. Pat. No. 5,625,964, issued to Lyden et al., discloses an athletic shoe having a sole with a rearfoot strike zone segmented from the remaining heel area by a line of flexion which permits articulation of the strike zone during initial heel strike of a runner. The line of flexion is located to delimit a rearfoot strike zone reflecting the heel to toe running style of the majority of the running population. In addition to allowing articulation of the rearfoot strike zone about the line of flexion, the sole incorporates cushioning elements, including a resilient gas filled bladder, to provide differential cushioning characteristics in different parts of the heel, to attenuate force applications and shock associated with heel strike, without degrading footwear stability during subsequent phases of the running cycle. The line of flexion may be formed by various ways including a deep groove, a line of relatively flexible midsole material, and a relatively flexible portion of a segmented fluid bladder.
Our prior patent, U.S. Pat. No. 7,383,647 (hereinafter “our prior patent”), which is incorporated by reference as though fully set forth herein, discloses a midsole for footwear comprising a medial element, which comprises a top plate, a bottom plate, and a plurality of strut members disposed between the top and bottom plates for supporting the top plate a distance away from the bottom plate. Adjacent strut members have a C shaped cross-section facing in the same direction. The midsole element may further comprise a heel cleft to increase the flexibility of the sole. In a preferred embodiment, the strut members on the medial side are arranged at an angle to the strut members on the lateral side of the sole. The directional design provides flexibility and stiffness anisotropically to the sole in the longitudinal and lateral directions of the sole respectively. In one embodiment, the midsole strut element on the medial side and the lateral strut element of the midsole element on the lateral side are integrally molded. In another embodiment, the medial and lateral top portions, medial and lateral shut members and medial and lateral bottom portions are independently selected from the following materials: thermoplastic polyurethane (TPU), polyester-TPU, polyether-TPU, polyester-polyether TPU, polyvinylchloride, polyester, thermoplastic ethyl vinyl acetate, styrene butadiene styrene, polyether block amide, engineered polyester, TPU blends including natural and synthetic rubbers, and blends or combinations thereof. In yet another embodiment, at least two adjacent medial struts are oriented at a first angle relative to a longitudinal direction of an article of footwear to define a medial stiffening axis, the at least two adjacent medial struts oriented and adapted to preferentially deflect in the same direction transversely to a medial stiffening direction in response to a force imparted on the medial element of the article of footwear during use. One of the at least two adjacent medial struts adapted to preferentially deflect in said same direction toward the other of said at least two adjacent medial strut members, thereby providing directional flexibility transverse to said medial stiffening axis.
The midsole disclosed in our prior patent may also include a lateral element comprising: a top lateral portion; a bottom lateral portion and a plurality of lateral strut members disposed between said top and bottom lateral portions for supporting said top lateral portion a distance away from said bottom lateral portion; wherein at least two adjacent lateral strut members are oriented at a second angle relative to the longitudinal axis of said article of footwear to define a lateral stiffening axis, said at least two adjacent lateral strut members oriented and adapted to preferentially deflect in the same direction transversely to said lateral stiffening axis in response to a force imparted on said lateral element of said article of footwear during use, one of said at least two adjacent lateral strut members adapted to preferentially deflect in said same direction toward the other of said at least two adjacent lateral strut members, thereby providing directional flexibility transverse to said lateral stiffening axis; wherein said lateral stiffening axis is arranged at an angle to said medial stiffening axis, said at least two adjacent medial strut members and said at least two adjacent lateral strut members adapted and arranged to provide flexibility and stiffness anisotropically to said midsole.